The present invention relates generally to method and apparatus for measuring the contrast sensitivity of the human eye and, more particularly, to such method and apparatus where contrast sensitivity, an indicator of visual function, is measured by determining a subject's response to luminance differences offered by a single edge, i.e., the luminance transition from a light area to a dark area.
In recent years, it has been recognized that detection of visual loss oftentimes is an early indication of eye disease. To this effect, a good and well executed test of visual function can prove to be a valuable diagnostic tool for the visual practitioner.
Perimetry is one type of visual test used for evaluating functions of the retina and the supporting neural network of the brain. In perimetry, the ability of the eye to perceive light at different retinal locations is measured. It has been found that when the retina or other portions of the visual system are impaired due to such diseases as optic neuritis, glaucoma or diabetic retinopathy, some loss of the ability of the eye to perceive light will occur. Perimetry is used to determined, as accurately as possible, those specific areas of the retina which have reduced sensitivity to small spots of light.
One example of a perimetry instrument currently available is the OCTOPUS.TM. automated perimeter which is distributed by Hitron Corporation of Norwood, N.J. The OCTOPUS automated perimeter employs a computer controlled hemispherical perimeter projection system which enables the entire visual field to be examined. A similar device is described in U.S. Pat. No. 3,705,003 to Lynn et al., which utilizes a computer to control the position, size and intensity of various test stimuli which are presented on a visual display screen at predetermined locations in the subject's visual field. The subject viewing the screen responds to each stimulus using a control device to indicate whether light was perceived.
Perception of light is but one of several functions of the retina/brain system, other functions including perception of pattern, motion and color. When the retina or some other portion of the visual system is impaired due to disease, some loss of one or all of these basic functions may be expected.
Recently, there has been an emphasis on the use of pattern perception to evaluate the status of the eye. This may be done by determining the amount of contrast in a pattern necessary for the pattern to be detected by the subject. The monitoring of "contrast sensitivity" is considered a useful early diagnostic tool because an eye with pathology will, oftentimes, lose its ability to perceive patterns, i.e. contrast, before it loses its ability to perceive small spots of light. Accordingly, visual pathology may affect one's contrast sensitivity before affecting a subject's ability to perceive light small spots of light, or their visual acuity.
Pattern perception represents a complex form of vision where information from the photoreceptors of the eye is summarized by the higher centers of the brain. Accordingly, in certain eyes with early pathology, this function will show the first indication of disease. Contrast sensitivity testing thus provides a very sensitive and early indication of the onset of eye disease and may be thought of as a type of window or probe to evaluate retina-brain function and/or status.
Contrast of a pattern is determined by the difference in luminance between two adjacent light and dark areas and may be measured quantitatively by measuring the luminance of the adjacent light and dark areas. Contrast sensitivity is a measurement of the amount of contrast necessary for the eye to perceive a pattern. While in perimetry the amount of light necessary to perceive a small spot is measured, in tests of contrast sensitivity, the amount of contrast necessary to perceive a simple pattern is measured. In testing for contrast sensitivity, a strategy must be employed to vary the contrast of a particular pattern in order to determine the minimum amount of contrast necessary to detect that pattern. Contrast threshold is the smallest amount of contrast necessary to detect the pattern and may be determined using a testing strategy similar to that used by the perimetry instruments.
An example of one commercially available instrument for measuring contrast sensitivity is marketed by Optronix of Evanston, Ill.
The current commercially available instruments used to test for contrast sensitivity suffer from a number of distinct disadvantages due, in part, to the bar test patterns which they utilize. Typically, the patterns used to determine the sensitivity of a subject consist of many vertical bars of alternating light and dark stripes. The size (width) of the stripes changes for different parts of the test. However, regardless of the stripe size, what is measured is the minimum amount of contrast necessary for the subject to correctly identify that the pattern was present. This is the contrast threshold. These patterns, known as "grating patterns", are all composed of bars without sharp edges, i.e. they appear "fuzzy". More formally the bars change from a light bar to a dark bar across a space in accordance with a sinusoidal function, and are frequently referred to as sinusoidal spatial frequency gratings. The term "spatial frequency" refers to the size of the bars, i.e. a large bar is high spatial frequency. Units of size are expressed in the number of cycles (light and dark bar parts) imaged on one degree of the retina, i.e. cycles/degree.
The equipment necessary to generate the requisite bar patterns and analyze the responses thereto is extremely sophisticated. Moreover, the current tests are time-consuming. It has been found that determination of eye function by any method other than a quick screening method is particularly tiring for the patient. Using such commercially available methods, the patient may be required to return to the clinic for repeated testing sessions. This has been found to be onerous for many patients.
A further problem with the instruments commercially available to test for contrast sensitivity is that the interpretation of the contrast sensitivity function is not well understood. For example, the medical or biological significance of a shifted peak or accelerated loss of sensitivity to large bars is not known.
In the commercially available devices for measurement of contrast sensitivity, the amount of contrast necessary to detect a grating pattern is determined for many different sizes of bars (spatial frequencies). The results are displayed so that contrast or threshold (expressed in decibels or percent) is plotted against spatial frequency on the x-axis of a graph. Comparisons to the normal or expected values are determined by point to point subtractions of the subject's values. This method does not provide an overall index of how well the subject's function compared against an expected normal function. It does suggest certain possible changes in the shape of the function, i.e., that a subject has lost more sensitivity to high spatial frequencies than, for example, to low spatial frequencies. However, these types of observations can be made about how the contrast sensitivity function differs from the expected, their medical importance is not well understood. Further, in many cases it is necessary to compare the measured results to results which would normally be expected from a typical observer. Unfortunately, such normal functions vary quite markedly from each other. Accordingly, precision or accuracy is lost with the current testing procedures for contrast sensitivity because uncertainty in the comparison function is high.